I consider myself reasonably knowledgeable when it comes to computer issues, but I've hit a wall, and I'm banging my head against it.

First, the background, so you understand the situation fully.

I built a computer for my mother roughly 2 years ago, with the following specs:
- AMD Athlon X2 4200+ (AM2)
- ASRock ALiveNF7G-HD720p
- 2x 2gb DDR2-800
- 1x 500gb Hard Drive
- 1x DVD-RW drive
- 1x Floppy drive
- 450w PSU
- 480w UPS
- Mid-Tower ASUS case
- 3x 120mm fans (one came with the case)

At first, I installed Windows Vista in it because my mother prefered it (she says the games, like Solitaire, look better). Since the specs were relatively high for her (internet browsing, occasional word processing, and listening to music) I went ahead with it.

It worked fine until a few months ago when my mother started to complaint about the music stutter, the mouse cursor having trouble moving, and everything was just SLOW.

My first thought was that it might be spyware, a virus, or something like that but, as much as I tried to "clean" her PC, it always went back to do the same. Now, I know some software can't clean every little piece of spyware and viruses out, so I went for a full system format.

Installed Windows Vista, updated it with the latest Service Pack, and... lo and behold, the problem surfaced always immediately. I swear I didn't even install anything in it, it was a fresh install, only with Windows updates, and nothing more.

Knowing about Windows Vista reputation for being the worst **** up since Windows ME, I was looking forward to the release of Windows 7 this month and, as soon as it was out, I got a copy of it.

Again, I went through the process of formating my mother's PC, install Windows 7, added AVG Internet Security, Adobe Acrobat Reader 8.5, and Windows Live Messenger - that's it.

Same problem. After a random amount of time, even if no one is doing anything, the computer starts to lag horribly.

I went into the Task Manager and tried to see what could be wrong. Immediately, I've noticed that both cores were pretty much maxed out! Bouncing up and down between 70 and 90%, but the weird part is that when you went to the "Processes" tab, there was nothing using that kind of CPU.

Going back to the "Performance" tab, I clicked on "Resource Monitor" and now I had something to go on... in the new window that opens, I could see that the huge amount of CPU being used was set to something called "System Interrupts".

I've googled for it and all I could find was a TON of people having the same problem as me, but no real solution.

Some suggestion I read were:
- Update Chipset drivers.
- Update Motherboard drivers
- Update Audio/Video drivers
- Turn off HD audio

I tried everything from above, nothing worked.

- Chipset drivers from nVidia official website - check, didn't solve the issue-
- Motherboard drivers from ASRock official site - check, didn't solve the issue.
- Audio/Video is on-board, so they were updated from the above sources - check, didn't solve the issue.
- Turning off the HD audio... the only way I know how to do this is to go into the BIOS, but then the computer has no sound at all.

I'm running out of options here...

So, I need your help. Any help!

First of all, what are "system interrupts"? What do they do? Why do I need them? Why they go crazy at random?

Second, why it didn't happen before, when I first installed Vista? Was it a Windows update that screwed things over?

Third, and most important, is there a solution to the problem that I may have missed?